Recently, many expectations are raised on an electric vehicle consuming less energy for traveling, and a power generation system using natural energy such as solar light and wind power due to prevention of global warming and concerns of exhaustion of fossil fuels. However, those technologies have the following technical disadvantages, which prevent their wide use.
Disadvantages of an electric vehicle are a low energy density of a driving battery and a short traveling distance per one time charge. On the contrary, disadvantages of a power generation system using natural energy are large fluctuation of a power generating amount, and necessity of a large capacity battery for output leveling, which results in high costs. Any of those technologies demand a secondary battery produced at a low cost and having a high energy density.
Here, a lithium ion secondary battery is expected to be applied to an electric vehicle and a power storage system because an energy density per weight thereof is higher than other secondary batteries like a nickel-hydrogen battery and a lead battery. However, a further highly improved energy density is required for responding to the demand of an electric vehicle and a power storage system. Such a further highly improved energy density requires an enhancement of energy densities of a cathode and an anode.
A layered compound having a layered structure assigned to a space group of R-3m and represented by the formula: LiM1O2 (where M1 represents metal elements other than Li) is widely used as a cathode active material that constructs a cathode of a lithium secondary battery. Metal elements other than Li generally include Co, Ni and their combination with Mn. Among them, a layered compound containing nickel (Ni) as a main component is known as a cathode material showing a high energy density. In particular, a layered compound having a high content of nickel over 70 atom % per metal elements (M1) may realize a reversible capacity over 180 Ah/kg.
Conventionally, proposed is a technology for improving battery properties of a lithium ion secondary battery having a cathode material made of a layered compound containing such a high content of nickel as a main component. For example, Patent Document 1 discloses a cathode active material used for a lithium ion secondary battery of which composition formula is represented by LixNi1-yMyO2+α (where 0.9≤x≤1.2, 0<y≤0.7, −0.1≤α≤0.1, and M is a metal).
Further, disclosed is that in such a cathode active material, a generating rate at a peak of H2O has a maximum value of 5 mass ppm/sec or less in the range from 200° C. to 400° C., and a generating rate at a peak of CO2 has a maximum value of 3 mass ppm/sec or less in the range from 150° C. to 400° C., when the cathode active material (e.g., 5 mg to 30 mg) is measured by the TPD-Ms.